Hybrid Electric Vehicles (HEV) are featured with an internal combustion engine and electric machines driven by state-of-art inverter systems. A high voltage battery is used in the electrified powertrain to supply power to the electric machines and to store energy recuperated during vehicle brake. The electric motor/generator(s) within a hybrid electric vehicle provides the vehicle with additional degrees of freedom in delivering the driver-demanded torque and may also be used to control the output speed of the engine. Three common HEV configurations are typically used in automotive industry, which are power split, series hybrid and parallel hybrid.
In the power split type hybrid electric vehicle, the electric generator and the internal combustion engine are interconnected by use of a planetary gear set, and the electric generator selectively provides a reaction torque which may be used to control (e.g., to reduce and/or augment) the speed of the vehicle's engine. In this manner, the generator is used to control the speed of the engine and cooperates with the planetary gear set and a traction motor to provide a continuous variable transmission (“CVT”) effect. One drawback associated with this type of hybrid electric vehicle arises from the limited amount of reaction torque which can be provided by the motor/generator.
The mechanical structure between the engine and the electric machine may become a mechanically constitute vibrating system during certain vehicle speeds. When the engine is an internal combustion engine, for example, a torque variation due to a gas explosion or reciprocating motions of the piston in the internal combustion engine causes torsional vibrations on the output shaft of the internal combustion engine and the rotating shaft of the motor. When the natural frequency of the shaft coincides with the forcible frequency, a resonance occurs. This may result in a foreign noise from the shaft and even fatigue destruction of the shaft in some cases. Such a resonance occurs in many cases at a revolving speed lower than the minimum of an operable revolving speed range of the engine, although it depends upon the type of the engine and the structure of the shaft.
The resonance of the torsional vibrations that may occur in the system at the time of starting or stopping the operation of the engine is observed not only in the power output apparatus but in any driving system, wherein the output shaft of the engine and the rotating shaft of the motor are mechanically linked with each other. The primary countermeasure against these troubles is that the output shaft of the engine and the rotating shaft of the motor are mechanically linked with each other via a damper. The dampers having a significant effect on reduction of the amplitude of the torsional vibrations, however, require a special damping mechanism. This increases the required number of parts and makes the damper undesirably bulky. The small-sized simply-structured dampers, on the other hand, have little effects.
The motor is generally under a proportional and integral gain (PI) control. In the procedure of outputting a torque from the motor to the output shaft of the engine and thereby positively stopping the operation of the engine, the I term (integral term) may result in undershooting the output shaft of the engine, which causes a vibration of the whole driving system. When the driving system is mounted, for example, on a vehicle, the vibration due to undershooting is transmitted to the vehicle body and makes the driver uncomfortable.
In general, the classical method to control hybrid electric vehicle speed control between the engine and electric machine may be done using one or more algorithms used to control the inertia of the engine and generator/motor by controlling the engine speed and/or the generator/motor speed. If any speed control happens to excite such a resonance between the engine and electric machine, serious speed and torque oscillation may be observed on the system. An improper hybrid electric speed control design may lead to engine speed/torque oscillation in classic engine speed control for HEV applications.